(1) Field of the Invention
The invention relates to the general field of semiconductor integrated circuits, more particularly to methods for planarizing the surface of a semiconductor wafer.
(2) Description of the Prior Art
As integrated circuits get formed, by depositing, and then shaping, a succession of layers, the outermost layer of the emerging structure becomes less and less planar. At some point this lack of planarity can no longer be tolerated and a planarization step is needed. A number of different methods for planarizing are in use in the industry, including chemical-mechanical polishing and liquid coating followed by freezing.
In the liquid coating method a liquid, such as a siloxane, which transforms into a glassy material on heating, is spread over the wafer surface by some standard method such as spin coating. Being a liquid, it finds its own level and settles after some period of time so that its upper surface is level, assuming that sufficient liquid was used to cover all protuberances on the surface of the wafer.
If excessively long settling times are to be avoided, liquids having relatively low viscosity have to be used. Unfortunately, in general, the lower the viscosity of the pretreated liquid the lower the density of the post-treatment glass. Since time is critical in a production environment, it is often the case that the liquid is not given sufficient time to fully settle so an excess is used and, following glass formation, this excess is then removed by chem-mech. polishing. This adds significantly to the cost of the planarizing step. Additionally, current methods for detecting just when sufficient glass has been removed by the chem-mech. polishing process are not entirely satisfactory and can lead to further loss of time or a reduction in final yield.
To avoid having to follow liquid planarization with chem-mech. polishing, it is necessary to add just the right amount of liquid to the wafer surface. This is not a problem, per se, but if insufficient time is allowed for the liquid to settle a number of defects can be introduced. For example, referring now to FIG. 1a, we show a schematic cross-section of a wafer substrate 1 on whose surface are a number of irregularites 3. Liquid layer 2 has been deposited over these with the intent of fully covering 3 and presenting a planar outer surface. Initially, before 2 has fully settled, its surface will be uneven, following approximately the countours of the underlying wafer, as shown.
In FIG. 1b a somewhat different scenario is represented. Although the surface of planarizing liquid layer 2 is more or less planar, settling is still not complete as a number of voids or bubbles 4 have been trapped in the liquid and more time will be needed for them to rise to the surface and disappear.
FIG. 1c is similar to 1a in that, initially, the surface of the planarizing liquid 2 roughly contours the surface of substrate 1, including protuburances 13. Since 13 are spaced relatively far apart, observations in regions of the wafer surface such as 5 could lead to the mistaken conclusion that the liquid had fully settled whereas liquid areas such as 7 (directly over 13) have not. FIG. 1d illustrates an extreme case of the scenario of FIG. 1c wherein liquid regions 8 not only still sit atop protuburances 13 but are on the verge of collapsing upon themselves to form voids such as 4 in FIG. 1b.
Thus, in order to deal with the problems outlined above, some means for reducing the time needed by the planarizing liquid to fully settle are sought. In the present invention the problem has been solved by the provision of artificial gravity as an assist to the earth's gravity during settling. We found somewhat different solutions to the problem in the prior art.
For example, Gupta (U.S. Pat. No. 5,348,615 September 1994) proposes the use of regelation as a means for temporarily reducing viscosity, thereby reducing settling time during planarization. The method works only for settling in very small areas and, more seriously, the class of materials that are subject to the regelation phenomenon is very small.
Ikeno (U.S. Pat. No. 5,264,246 November 1993) teaches a modified spin coating method wherein rotation of the wafer is combined with tilting it so that some centrifugal force is also applied to the wafer. An inherent difficulty associated with this approach is that it is subject to a shadowing problem--planarizing liquid moving parallel to the wafer surface is shadowed by the various irregularities on the surface and builds up on the sides that face the center. As a result, an excess amount of the liquid must be used to ensure that everything gets covered and the subsequent chem-mech. polishing step is still needed.